This invention relates to an optical assembly that employs a single element lens to view an indicia target with an image sensor. More particularly it relates to an optical assembly in a scanning instrument that minimizes field curvature of indicia being read with a single element lens, and projects an image of the indicia onto an image sensor.
Most modern optical scanners and bar code readers employ optical systems to direct reflected light from the indicia being read to an image sensor. Today multi-element CCD arrays are commonly used as the image sensor, and, with such array, a bar code symbol can be read while maintaining the reader in a stationary position, or by moving the reader with respect to the indicia.
In such scanners it is desirable that the operational depth of field be maximized while giving appropriate attention to the optical efficiency of the lens imaging system. In an optical system attention should be given to many of the following issues; optical efficiency; well known lens faults such as spherical and chromatic aberration; minimization of image field curvature, such that a relatively wide symbol can be adequately viewed; and the well known cosine to the fourth power illumination falloff phenomenon. Dealing with these issues under the constraints of adequate depth of field and maintaining economy in materials and manufacturing imposes severe burdens on those striving to advance the bar code reading art.
In the prior art, correction of lens aberrations has been achieved by multi element lens systems. Representative of this approach is U.S. Pat. No. 4,766,300 to Chadima, Jr. et al, in which a bar code reader optical assembly employs five lenses constructed of polystyrene and acrylic lens material. This system further employs mirrors to establish a long, filed optical path in order to achieve suitable depth of field and image resolution for a bar code application.
In such systems, the optics are mechanically complex and expensive to fabricate and cling.